Do you have an article that gets it "right"? I am not sure what there is to worry about, but whatever the ppi, if we are talking about something above 300, that sounds pretty good to me. CNET (
http://news.cnet.com/8301-13579_3-57567519-37/ipad-mini-retina-display-may-surpass-ipad-4/), by the way, is giving the same number. It may be that they are reporting what they are hearing. At this point, I trust whatever imaginary source they have more than our calculations, which are not based on any knowledge of what is actually being produced.
You don't get it either. Apple chose the resolution (# of pixels) to match iPad 1 and 2 so apps won't need different assets (think the images for buttons, pictures, game art, etc.). Apple chose 7.9" physical screen size (as opposed to 6" or 7" or whatever) so that the pixel density is the same as iPhone 3GS. This means that GUI elements (like the actual physical size of buttons, icons, etc. NOT pixel size but size measured with a ruler) will be the same size that users are already used to touching on their current iPhones and iPads. This is incredibly smart because the roadmap to retina is already built by their iPhone 4, 4S, 5. They can order the same exact LCDs as iPhone 4, 4S, 5 but just cut them at larger sizes. This helps with supply chain. They don't need to deal with a new problem of "how to go to retina?" because the attributes of the non-retina display is the same as their previous non-retina-but-was-upgraded-to-retina devices. Albeit just larger physical size.
iPhone 3GS has 163 PPI. To go retina, Apple doubled the pixel count both horizontally and vertically. Result was iPhone 4 which has 326 PPI. Notice 163 * 2 = 326.
iPad 1 and 2 has 132 PPI. To go retina, Apple double the pixel count both horizontally and vertically. Result was iPad 3 which has 264 PPI. Notice 132 * 2 = 264.
iPad mini has 163 PPI. To go retina, Apple needs to double the pixel count both horizontally and vertically. Result will (hypothetically) be iPad mini with Retina which has 264 PPI. Notice 163 * 2 = 326. To go through with this, they just need to order the same exact part as iPhone 4, 4S, 5 but just larger physical size. You don't need a totally different supplier or totally different part. It's the same exact part with same exact PPI, just larger physical size.
Apple tends to use the same core components (CPU/GPU, Bluetooth, WiFi, GPS, Accelerometer, Gyro, camera sensors, screen panel/technology/type/etc.) across various iOS devices. This holds many advantages. It's easier to maintain a performance standard since it's the same component just different form factor. It's easier to write apps for since you know the WiFi chip isn't from 10+ vendors but from 1-2. It's also good for scaling production of devices since they shared core components. You don't need to order a whole bunch of parts for each device. Just the same one but a massive quantity. It's also good for introducing new models because core technologies are leverage and shared from past.